1. Field of the Invention
This invention pertains generally to composite material and more specifically a technique for the ingress and egress of fiber optic sensor leads from the surface of composite materials.
2. Description of the Related Art
The purpose of embedding fiber optic sensors in materials is to accurately measure a specific material property at a specific location in the composite specimen. To accomplish this, the fiber optic sensor (FOS) must be placed precisely in the desired location (including depth or layer), and ingress of the optical fiber leads must be accomplished with a minimum of risk to the optical fiber and host composite part.
When a fiber optic sensor strand is embedded within a composite part, it is necessary to provide a lead through which the sensors can be interrogated. This lead extends out of the part, and must be of sufficient length and in good condition for cleaving and splicing operations to standard fiber optic connectors. The fiber lead is fragile, being made of glass, with a diameter, generally, of 125 microns (0.005 inches). This lead serves as the only link between the embedded sensors and the readout electronics, if the lead suffers damage then the sensors are lost, as they are irretrievably buried within a high strength composite component.
Cure process for many composite materials is an extremely harsh environment for the fiber optic sensor, and the sensor lead. Most high performance composites are consolidated under high pressure and temperature. Pressure is applied often through the use of a vacuum bag, hard tooling with an expanding mandrel, shrink tape, or a combination of the above. Failure of the lead can occur during the application of pressure due to pinching or kinking of the lead between parts of the tooling, due to relative motion of the parts of a given cure fixture. The lead does not have to break to fail; permanent sharp bends (kinks) in the fiber optic lead will render it useless as a waveguide. Likewise, subsequent post cure operations. such as disassembly of the curing fixture, are extremely hard on fiber optic sensor leads due to the tendency of the composite material to bleed or leak resin during the cure process.
Another less obvious mode of failure of the lead occurs when an improper tubing schedule is used to protect the leads at the ingress point. Without proper damming, resins will flow up the tubing during the cure cycle through capillary action. If the tubing is sufficiently oversized, air bubbles form within the tubing around the fiber optic cable. Upon later flexing of the lead the fiber optic cable can break at these bubbles, particularly if the tubing is made of such a material as Teflon.RTM..
The last major mode of failure is breakage of the lead during handling and machining operations on the composite part. Leads which are not routinely armored and not sufficiently strain relieved will suffer damage under normal handling and machining operations.
The most widely used method for ingress and egress of the fiber optic leads are from the edge of a part. this method has been used successfully for test coupons but has limited practical applications because the edge of the coupon cannot be machined without chopping off the fiber optic cable in the process, or leaving an un-machined portion of material around the edge. Also, the fiber optic sensor lead is prone to breakage or severe kinking at the edge of the laminate during vacuum bagging as it is unsupported. Further, if the lead is supported to avoid the previously noted deficiency, then resin from the composite part often flows over the lead, which generally is spooled up at the edge of the laminate. This often causes fiber breakage during de-bagging or mold disassembly; the resin glues the fiber coil to itself and to parts of the mold or vacuum bag assembly. Interlaminar stress concentrations in composite parts becomes extremely large at the edge of a part. Test coupons exhibit this by edge delamination prior to failure. A discontinuity caused by a fiber optic cable at this area makes this condition worse as it acts as a stress riser or defect.
Egress from the surface of a part between layers of vacuum bagging materials has caused the following problems. The fiber left a deep imprint on the part surface, and thus created a defect and a possible failure initiation point. Resin flow into the vacuum bag material, i.e., breather (a material that resembles quilt batting) around the ingress-egress point creates a poor housekeeping condition, and the fiber optic cable must be carefully picked out of the resulting resin buildup. The fiber is unprotected from kinking at the egress point during vacuum bagging/mold assembly during this process. Egress points are unprotected after cure, no strain relief devise is used to protect the leads.